A variety of medical procedures involve electrically monitoring and/or stimulating neural tissue, such as regions of the cortex or spinal cord. For example, epileptogenic foci localization may be accomplished through cortical monitoring procedures; and various neurologically based pain conditions may be treated with cortical or spinal stimulation. Electrical signals may be exchanged with neural tissue through an electrode that includes a set of electrically conductive contacts.
The effectiveness of a neural stimulation procedure may be related to the electric field distribution produced by or associated with an electrode employed in the procedure. In general, the electric or stimulation field distribution depends upon a) electrode design; b) the particular electrode contacts to which electrical stimulation signals are applied; and c) the magnitudes and polarities of applied stimulation signals. An electrode's design encompasses the structure and spatial organization of its contacts, and/or the as-manufactured electrical couplings thereto. In order to maximize the likelihood that neural stimulation will be effective, an electrode design should be capable of producing an intended or desired type of stimulation field distribution. Depending upon stimulation requirements, an electrode design capable of providing flexibility with respect to manners in which stimulation field distributions may be established, configured, or tailored may be advantageous.
Neural microelectrodes are designed for micro-scale neural monitoring and/or stimulation, that is, highly localized signal exchange with very small neural populations or single neurons. Neural microelectrode types may include patch clamp or pipette microelectrodes; etched and/or micromachined needle electrodes or probes; and annular microelectrodes. An annular microelectrode capable of preferentially stimulating a single neuron soma is described in U.S. Pat. No. 5,411,540. Unlike the procedures disclosed in U.S. Pat. No. 5,411,540, many neural monitoring and/or stimulation procedures involve signal exchange with sizeable neural populations, i.e., hundreds, thousands, many thousands, or even millions of neurons. The microelectrodes disclosed in U.S. Pat. No. 5,411,540 accordingly have very limited applicability to such procedures.
Neural microelectrode arrays include multiple neural microelectrodes organized in a regular pattern and formed or mounted upon a substrate. Although a neural microelectrode array may be capable of monitoring and/or stimulating a larger neural population than an individual neural microelectrode, such an array may be undesirably complex and/or expensive from a manufacturing standpoint.
Grid electrodes may facilitate macro-scale neural monitoring and/or stimulation, that is, neural tissue monitoring and/or stimulation involving hundreds, thousands, hundreds of thousands, or perhaps millions of neurons. FIG. 1 is a plan view of a conventional grid electrode 100, which comprises a plurality of contacts 110 uniformly arranged in an array or a set of generally rectangular or rectilinear patterns; a lead wire 120 coupled to each contact 110; one or more electrode leads 130 into which lead wires 120 may be organized and/or routed; and a medium, substrate, or backing 140 upon and/or within which the contacts 110, the lead wires 120, and possibly portions of the electrode leads 140 reside. Conventional grid electrodes 100 are available from Ad-Tech Medical Instrument Corporation of Racine, Wisc. In general, the contacts 110, the lead wires 120, one or more portions of the electrode leads 130, and the substrate 140 are formed from biocompatible materials in a manner readily understood by those skilled in the art.
Conventional grid electrodes 100 may include a significant number of contacts 110. Such grid electrodes 100 maintain a one-to-one ratio between the number of contacts 110 and the number of lead wires 120. Thus, a conventional eight-by-eight grid electrode 100 having sixty-four contacts 110 includes sixty-four lead wires 120. Any given lead wire 120 may be coupled to a desired stimulation signal via an external signal routing interface that is connected to a stimulation signal source in a manner readily understood by those skilled in the art. Conventional grid electrodes 100 may facilitate a limited degree of simulation field configurability through selective coupling between specific contacts 110 and particular stimulation signals.
An electrode implant procedure may be highly invasive from a surgical standpoint, possibly requiring, for example, a craniotomy. Electrode reliability is therefore of paramount importance. Unfortunately, the large number of lead wires 120 resulting from a grid electrode's one-to-one contact to lead wire ratio increases the complexity and decreases the reliability of an electrode lead 130. Thus, conventional grid electrode arrays may not be suitable for use in procedures that require implanted electrodes.